Organic halogen compounds are widely used to impart flame retardancy to organic polymers.
Organic halogen however do generate highly corrosive halogen based compounds and other gases during combustion, develop a dark smoke under a fire scenario, and are becoming a topic of environmental concern, so that halogen free flame retardants are gaining more and more attraction.
Metal hydroxides, in particular Aluminum hydroxide and Magnesium hydroxide, are non toxic and non corrosive flame retardant fillers for polymers, they generate low smoke when burned and for these reasons they represent the ideal choice for environmentally friendly compounds, or for critical application like zero halogen cables. Their flame proofing effect is based on the endothermic decomposition and further interaction of the oxide residue within the polymer, leaving a “protective char”.
Main application for metal hydroxides is in wire and cable, basic polymers used are low density polyethylene (LLDPE), Ethyl vinyl acetate (EVA), and Polyolefins. Experience show that a filling level of around 55% by weight in EVA copolymer containing 18% Vinyl Acetate is generally necessary to fullfill the basic flame retardancy requirements in some applications. However, very often in order to meet stringent standards, far higher values are required, for example up to 65% of filler.
One drawback in the use of metal hydroxides is clearly the high filling level which is required, that often results in reduced mechanical properties as well as reduced extrusion efficiency due to increate processing viscosities. Therefore, a lot of work has been spent into the improvement of either the efficiency of metal hydroxides as flame retardants and on the processability and mechanical properties of such compounds by adjusting the metal hydroxide properties (surface treatments, improved morphology) as well as by adjusting the formulation (use of polymer-filler coupling agents and processing aids additives).
One approach, in order to improve flame retardant performances and physical properties of metal hydroxide filled polymers is to include co-additives (also called synergies) into the polymer formulation.
The use of the so called “char forming” additives in polyolefins and rubber as a synergic to metal hydroxides has been previously described.
For instance, the use of zinc borate, which is able to improve overall flame retardant properties is well know, as reported for instance in “Recent avances in the use of zinc borate in flame retardancy of Eva” (Polymer degradation and Stability 64 (1999) 419-425).
Also metal molybdates have some efficiency in promoting char forming, as reported for instance in “Enhanced FR performance enabled by magnesium hydroxide with metal molybdate in EVA” (Wire and Cable compound Proceedings of the 58th International Wire and Cable Symposium, pag. 569-576).
Poly Di Methyl Siloxane (PDMS), also called silicones or silicon oils or silicone gums or rubbers, as such or in masterbatches form are regularly used to improves processing of hydrate filled compounds and improve surface appearance. On the top of their processing aid function, PDMS are also know to act as flame retardant synergic, as reported for instance in U.S. Pat. No. 4,731,406, EP0466193, EP0402904.
More recently, organic metal phosphinate used with alpha-olefins/vinyl acetate having a defined vinyl acetate content and metal hydroxides are showed to increase flame retardant performance, see for example WO 2011/076760 assigned to Lanxess. According to WO 2011/076760, a combination of Metal hydroxides and organic metal phosphinate is limited to polymers composed by alpha-olefins/vinyl acetate with vinyl acetate content range from 40% to 90% by weight. Alpha-olefins containing vinyl acetate lower than 40% by weight are more difficult to make flame resistant, even though they are commonly used in cables compounds because of their benefits in processing and mechanical properties. There is therefore the need of improving flame retardancy of metal hydroxide containing compositions even in a broader range of vinyl acetate content.
Flame Retardancy Evaluation
Flame retardant properties may be well evaluated through cone calorimeter measurements.
The cone calorimeter (ASTM E1354/ISO 5660) has long been a useful tool for fire safety engineers and researchers interested in quantitative material flammability analysis. It remains one of the most usefull bench-scale tests that attempts to simulate real-world fire conditions.
The cone calorimeter brings quantitative analysis by investigating parameters as:                HRR=Heat Release Rate (kW/m2)        TTI=Time To Ignition(s)        THR=Total Heat Release (kW/m2)        
For example, the peak HRR is an important parameter, that can be used to measure the intensity of fire. Sometimes one or more selected measurements provide useful information in regulatory fire scenarios, and the most relevant specific example are FIGRA or FPI:                FIGRA=fire growth rate (kW/m2s)=peak HRR/time to peak HRR        FPI=fire performance index (m2s/kW)=TTI/peak HRR        
The higher the value of the FPI or the lower the value of FIGRA, the higher would be the product safety rank. Despite there is not widespread agreement in the industry about which parameter is most meaningful, cone calorimeter on cable specimens has been successfully correlated with real-scale burning tests.